Generally, in this type of image forming apparatus, an electrostatic latent image is formed on an image carrier based on input image data, that electrostatic latent image is developed (made manifest) by a developing apparatus, and a development image is formed on the image carrier. The development image formed on this image carrier is transferred to a transported sheet, and afterwards the development image is fused by a fixing apparatus with heat and pressure and fixed to the sheet.
Incidentally, when a positional shift occurs between the development image formed on the image carrier and the transported sheet due to transport shift of the sheet or the like, the position of the read original image and the position of the formed image will differ. Often this transport shift of the sheet is not constant, and varies according to differences in the size and type of sheet, or the housing means in which sheets are contained. Accordingly, if a positional shift occurs between the development image and the sheet when performing borderless image formation, in which an image is formed on the entire sheet (full-sheet image formation), loss occurs in the image transferred to the sheet, and the image formed will be very unsightly.
Thus, considering positional shift between the development image formed on the image carrier and the sheet due to transport shift of the sheet, a large image (development image) is formed with a certain margin on the image carrier, and even if transport shift of the sheet occurs, a good image without loss can be formed.
However, with the configuration as stated above, when forming an image on the image carrier that greatly exceeds the size of the transported sheet, the amount of developer recovered by the cleaning means after not being transferred to the sheet increases, and with an image forming apparatus that cannot reuse the recovered developer, in addition to being very uneconomical because the recovered developer is discarded, the cycle until the recovered developer is full becomes very short. Also, as shown in FIG. 25, in a cleaning means “c” provided with an integrated container “a” that recovers developer “t” on a transfer belt “d” that attracts and transports a sheet “p”, that recovered developer “t” is partially uneven and much of it accumulates in the area located in the left and right edge portions of the sheet “p” perpendicular to the sheet transport direction, so that it becomes easy for a portion of the recovered developer t to leak out, leading to poor cleaning.
Accordingly, conventional technology is known (for example, see JP 10-186951A) wherein upstream in the sheet transport direction relative to the transfer point where the image formed on the image carrier is transferred to the sheet, a detecting means is provided that detects the position of the edge portion of the sheet transported towards the transfer point, the size of the image on the image carrier is determined based on the size of the sheet whose edge position has been detected by this detecting means, and by forming an image on the sheet according to the image formed on the image carrier after that size determination, an image is formed on the image carrier whose size matches the size of the transported sheet, and the amount of developer recovered by the cleaning means after not being transferred to the sheet is reduced.
However, in the conventional technology described above, because an image is formed on the image carrier after detecting the edge portion position of the sheet with the detecting means and determining the size of the image on the image carrier, it is necessary to provide the detecting means far upstream in the sheet transport direction relative to the transfer point. Thus, the sheet transport path from the detection point of the sheet edge portion position detected by the detecting means to the transfer point must be quite long in the sheet transport direction, and the size of the image forming apparatus becomes quite large. Furthermore, when the sheet transport path from the detection point of the sheet edge portion position detected by the detecting means to the transfer point is lengthened, the time necessary for image formation is lengthened correspondingly.
Also, when the sheet transport path from the detection point of the sheet edge portion position detected by the detecting means to the transfer point is lengthened, although there is a risk that loss will occur in the image on the sheet due to transport shift of the sheet because the detection accuracy of the sheet edge position detected by the detecting means decreases, if the detecting means is moved closer to the transfer point, image writing to the image carrier will have already begun at the point in time that the sheet edge position is detected, and determination of the image size on the image carrier is not performed in time.
Incidentally, upstream in the sheet transport direction relative to the image carrier, a registration means is provided that is a timing matching means for matching the position of the image formed on that image carrier and the transported sheet. After once stopping the transported sheet, the timing is matched and then sheet transport is restarted. This registration means not only has a function that matches timing with the image on the image carrier, it also has a function that corrects skew (tilted transport) of the transported sheet.
However, there is a limit to the function of this registration means that corrects skew, and for skew that exceeds a tolerance, more or less tilted transport of the sheet sometimes occurs without skew being completely corrected. In that case, in order to judge the skew state of the sheet, because detection of the edge position must be performed at multiple places in the transport direction for the transported sheet, the position of the detecting means must be further separated upstream in the sheet transport direction in order to judge the skew state of the sheet at a timing earlier than the formation of the image on the image carrier. When this detecting means is located downstream of the registration means in the sheet transport direction, the image forming apparatus must be made even larger.